A conventional method of focusing control obtains a focusing error signal by introducing astigmatism into the light reflected from the disk by using a cylindrical lens, and utilizing the fact that the shape of the reflected light beam on a four-quadrant light detector changes due to the surface deflection of the disk. The following will explain the conventional art by referring to FIGS. 9 to 11. In the following explanation the same elements are shown by the same symbols.
FIG. 9 is a block diagram illustrating schematically one example of a conventional optical recording and reproducing equipment which incorporates a focusing control device. The optical system consists of a light source 1 of a semi-conductor laser, condenser lens 2 to condense the light beam from the light source 1, cylindrical lenses 3 and 4 which have a lens effect only in one plane to compensate for the light beam from the light source 1 having different spreading angles in the vertical and horizontal directions, polarizing beam splitter 10 arranged between the lenses 3 and 4, reflecting mirror 5 to reflect the light from the light source 1 and disk 8, and a magnetic diaphragm lens driving device 9 which is similar to the voice coil of a speaker and moves the diaphragm lens 6 by means of a servo control system to follow the surface deflection of disk 8 so that the focus of the small spot of light is always on the disk 8.
The disk 8 is rotated by the disk motor 13. The beam splitter 10 refracts the light reflected from the disk 8 as illustrated, and the convex lens 11, in cooperation with the convex cylindrical lens 4, irradiates the reflected light on the light detector 12 so as to introduce astigmatism into the reflected light. That is, the focal point of the light reflected from the disk 8 in the plane of light where the convex cylindrical lens 4 is effective is at position a indicated in solid lines and the focal point of the light reflected from the disk 8 in the plane of light where the convex cylindrical lens 4 is not effective is at position b indicated in broken lines. Since the reflected light is condensed at different points as aforementioned, the light detector consisting of four quadrants detects the change in the shape in accordance with the surface deflection of the disk 8.
FIG. 10 is an explanatory figure to show the shapes of the reflected light pattern received by the four light receiving areas of the light detector shown in FIG. 9 and the change in shape of the reflected light pattern on the light detector 12 in accordance with the surface deflection. FIG. 10(b) shows the reflected light pattern when the distance between the diaphragm lens 6 and disk 8 is a desired length and the small spot is formed on the disk 8 and the reflected light is almost circular on the light detector 12. FIG. 10(a) and (c) show the reflected light patterns on the light detector 12 when the distance between the diaphragm lens 6 and disk 8 is shorter or longer than the desired length.
If the quantities of light received by quadrants A, B, C and D are P.sub.A, P.sub.B, P.sub.C and P.sub.D, respectively, the focusing error signal Ue will be: EQU Ue=(P.sub.A +P.sub.C)-(P.sub.B +P.sub.D) (1)
and when Ue=0 the small spot is formed on the disk.
FIG. 11 is an explanatory figure to show the change in characteristics of the focusing error signal Ue in accordance with surface deflections of the disk 8, and the relative distance between the condenser lens 6 and disk 8 is shown on the axis of the abscissa and focusing error signal Ue on the axis of the ordinate. The axis of the signal Ue can be considered to be the surface of the disk. The focusing error signal Ue is given by the above formula (1) and is proportional to the quantities of light P.sub.A through P.sub.D received on the four light receiving quadrants A through D of the light detector. Therefore, in comparison with the focusing error signal as shown by the symbol a (the characteristic when there is no surface deflection) in FIG. 11, the focusing error signal when the disk 8 approaches the objective 6 due to surface deflection increases as shown by the symbol b because the reflection to the objective 6 increases. If the quantities of received light P.sub.A -P.sub.D relatively decrease because the amount of reflection to the objective 6 decreases or the lens is contaminated, the focusing error signal Ue decreases as shown by the symbol c in the figure.
Therefore, in such a focusing control as aforementioned, the focusing error signal is changed when the reflectance of the disk changes, when there is a considerable change in the quantity of reflected light due to the continuous emission of the laser light during reproduction, recording or erasing of the optical magnetic disk, when there is a change in the quantity of light due to the power correction in the radial direction of the disk, or when the quantity of light incident upon the light detector is changed due to contamination or deterioration of the optical parts. For this reason, it has the disadvantage that the total gain within the focusing control loop is changed. If the quantity of reflected light decreases, the gain decreases and the accuracy with which it follows the surface deflection is deteriorated, and if the quantity of reflected light increases, the gain increases and the system becomes unstable and oscillation tends to occur.
As to the tracking system, the tracking error signal Ute is: EQU Ute=P.sub.B -P.sub.D ( 2)
and therefore, the system becomes unstable because the loop gain of the servo system is increased or decreased due to the change in the quantity of light.
In order to eliminate the aforementioned disadvantages, such device as shown in Japanese Patent Journal No. 56-148745 has been proposed. But that device uses a FET as a variable resistance and the FET characteristics have a high dispersion, and even if the devices are formed under the same constant conditions, adjustment is required. Also, even if a constant voltage is applied between the gate and source, the transfer function of the circuit system is changed due to the change in ambient temperature, etc., and the DC drift of the output taken out of the drain side is large and therefore it is difficult to put it to practical use.